We will study electrophysiological and morphometric aspects of the postnatal development of the rabbit auditory cortex before and after the onset of hearing. In animals prior to hearing onset, the location of the auditory cortex in each animal will be determined electrophysiologically by using microelectrodes to record evoked potentials in response to electrical stimulation of the medial geniculate or inferior colliculus. After the onset of hearing, we will study the electrophysiological characteristics of the developing auditory cortex by recording averaged evoked potentials and multiunit activity in response to acoustical stimulation. The brain of each animal so studied will be processed according to Golgi-Cox Nissl procedures. The dendrite systems of randomly selected auditory neurons at each period will be analyzed with a newly developed computer microscope. We will provide new information on: 1) the time course of development of acoustically evoked cortical activity and the formation of the cortical representation of the cochlear partition, 2) the postnatal growth of radial (dendrite length and number) and spatial (orientation and dispersion) characteristics of dendrite systems in the auditory cortex, and 3) the relationship at each stage of development between the morphology of cortical neurons and the electrophysiological properties of the auditory cortex. Results from the proposed research will provide quantitative information on the postnatal sequence of electrophysiological and morphological development in the audtiory cortex, a sensory cortex which has received little attention in developmental neurobiology.